A water treating device of above-mentioned type in the prior art is disclosed, for example, in Japanese patent publication No. 54638/1983.
The water treating device is constituted in that a water wheel is rotatably fixed on upper side of a treating tank, a circulation partitioning plate with lower side opened is installed from just below a water raking-out member of the water wheel throughout the whole width region of the treating tank, the water level is adjusted so that a part of the water raking-out member of the water wheel is sunk in the water, water circulation is generated from the circulation partitioning plate as the border by rotating the water wheel, and an immersion filter bed is arranged at least on one side of the circulation partitioning plate.
The immersion filter bed is formed by contact materials such as Rasching ring, plastic short tubes in net shape or small stones (gravel), filled or suspended.
In the filter bed constituted by the contact materials, however, if the specific gravity of the contact material is more than 1.0, the voidage is gradually decreased towards the bottom of the treating tank. On the other hand, if the specific gravity is less than 1.0, the voidage is gradually decreased towards the top of the treating tank. Consequently, it is difficult to obtain the contact flow of suitable rate for adhering and activation of microorganisms (If too slow, the microorganisms cannot be adhered; if too rapid, oxygen supply becomes insufficient), and to secure the voidage which cannot be closed by flocks or waste throughout the whole filter bed. If closing of the immersion filter bed is produced, the filter bed must be washed. However, the washing is troublesome and not preferable because the microorganisms are washed away (Time is required for the microorganisms to be adhered to the filter bed again).